1. Field of the Invention
The present invention relates to a range finder for obtaining measured distance information by emitting light to a photographed subject and detecting reflected light from the photographed subject in synchronization with the light emission. More particularly, the present invention relates to a device for effectively emitting light from a light emitting element in the range finder used in a camera.
2. Description of the Related Art
A so-called active type range finder is known as a range finder used in a camera. In this active type range finder, measured distance information is obtained by operating a light emitting element, emitting light to a photographed subject, detecting reflected light from the photographed subject and processing this reflected light. A distance from the camera to the photographed subject is detected by this measured distance information.
The range finder of this kind is generally divided into two systems composed of a synchronous detecting system and a one-shot system. In the synchronous detecting system, the light emitting element is intermittently operated and the reflected light is detected in synchronization with this operation of the light emitting element. The reflected light is synchronously integrated so that the distance between the camera and the photographed subject is detected. In the one-shot system, the light emitting element is operated with one shot pulse so that one measured distance information is obtained by one light emission.
An infrared light emitting diode is generally used as the light emitting element. This infrared light emitting diode is operated with a driving electric current of several 100 mA in the synchronous detecting system and a driving electric current of about several A in the one-shot system so as to obtain sufficient light emitting power. In the range finder of the camera, a battery is used as a power source for operating the above infrared light emitting diode.
However, the battery is consumed by use so that a voltage of the power source is reduced.
Further, in a low temperature environment, etc., output resistance of the battery is increased so that it is difficult to output a large electric current having a predetermined voltage. In contrast to this, the number of kinds of infrared light emitting diodes capable of obtaining a sufficient light emitting amount with low voltage is small and the infrared light emitting diodes are expensive.
It is sufficient to set a battery voltage to about 1.5 V to 2.0 V so as to operate an infrared light emitting diode by the synchronous detecting system. However, in the one-shot system, it is necessary to set the battery voltage to be equal to or higher than 2.5 V. Therefore, it is normally difficult to select the infrared light emitting diode to obtain a sufficient light emitting amount in the one-shot system in comparison with the synchronous detecting system. In general, infrared light emitting diodes have no problem in the synchronous detecting system, but have a problem in the one-shot system.
Recently, the focal length of a zoom camera has been increased. Therefore, distance measurement of a photographed subject at a long distance is indispensable to the camera. Light must be suitably emitted to the photographed subject at a long distance so as to precisely measure the long distance. Therefore, it is necessary to fulfill performance of the light emitting element at its maximum in the synchronous detecting system.
The following techniques are generally known as a technique for fulfilling the performance of the light emitting element at its maximum.
(1) The voltage of a low voltage direct current power source is increased by using a DC-DC converter, etc. so that a voltage applied to the light emitting element is increased. PA1 (2) The voltage applied to the light emitting element is increased by using a voltage doubler circuit. PA1 (3) A high output is stably obtained by operating the light emitting element with a constant electric current. PA1 the range finder comprising: PA1 the constant current control means is constructed by a constant current circuit comprising: PA1 the constant current control means is constructed by a constant current circuit comprising: PA1 the constant current control means is constructed by a constant current circuit comprising:
For example, Japanese Patent Application Laying Open (KOKAI) No. 63-171313 shows a technique using the voltage doubler circuit. In this technique, a capacitor for light emission is charged at a battery checking time. A sum of the charging voltage and the voltage of a power source is applied to the light emitting element at a light emitting time.
Concretely, a device shown in the above Japanese Patent Application Laying Open (KOKAI) No. 63-171313 has a direct current power source and a first series circuit constructed by a capacitor for light emission and a dummy load for battery check connected to this direct current power source. This device also has a capacitor forming a stabilizing power source and connected in parallel to this first series circuit through first switching means. This device also has second switching means interposed between one end of the direct current power source and a connection point of the capacitor for light emission and the dummy load for battery check. This device further has a second series circuit composed of third switching means and a light emitting element interposed between the other end of the direct current power source and a side point opposite to the connection point of the capacitor for light emission. When a battery is checked, the first and second switching means are closed and the capacitor for forming the stabilizing power source is charged by a sum of the voltage obtained by the direct current power source and the charging voltage of the capacitor for light emission. At a distance measuring time, the second and third switching means are closed and the above voltage sum is applied to the light emitting element.
For example, Japanese Patent Application Laying Open (KOKAI) No. 62-156631 shows a technique for operating the light emitting element with a constant electric current. In this technique, the light emitting element is operated with a constant electric current in accordance with a charging voltage of a capacitor for light emission.
Further, Japanese Patent Application Laying Open (KOKAI) No. 1-199109 shows that reliable measured distance information is obtained by emitting light from the light emitting element plural times.
However, the above-mentioned general methods have the following disadvantages.
In the above method (1) for increasing the direct current voltage, cost of parts for increasing this direct current voltage is taken and a wide space for mounting these parts is required. Useless energy is consumed to increase the direct current voltage. Further, it takes time to set the direct current voltage to a predetermined voltage. Accordingly, this method is not suitable for a case in which a distance measuring operation is performed by continuously emitting light from the light emitting element as in the synchronous detecting system.
In the above method (2) using the voltage doubler circuit, there is a problem in that no electric current flowing through the light emitting element is constant. Further, a voltage of the capacitor for light emission depends on a voltage of the direct current power source for charging this capacitor. Therefore, it is difficult to stably obtain a high output in a wide range of the power source voltage.
In the above method (3) using a constant current operation of the light emitting element, no predetermined electric current can flow through the light emitting element when light is emitted from the light emitting element under a worst condition of the direct current power source (when an internal resistance of the direct current power source is increased with low voltage in a low temperature environment, etc.).